Where flexible printing forms are used, the term flexographic printing, in which low-viscosity inks are applied to the elastic printing plate material via an inking roller or an anilox roller, is used. Being a relief printing method, the raised areas of the printing form are image bearing, with the actively printing areas standing proud.
In printing presses, primarily flexographic printing presses, in which the printing units are constructed from at least three rotating cylinders, the printed image is uniformly created on the substrate to be printed, that is the printed material, in that in a first nip ink is transferred from an inking roller to the raised regions of a printing form bearing roller, the printing form carrier. In a second nip the ink is then transferred from these raised regions of the printing form carrier to the substrate. The substrate here is supported by a substrate-bearing roller, referred to as the counterpressure roller. The raised printing form here conjointly rotates with the substrate-bearing roller and enters and exits the first and second nip in a cyclical manner.
The printing form on the printing form carrier has a run-up edge and a run-off edge which constitute the beginning and the end of the motif to be printed, respectively. There is spacing between the run-up edge and the run-off edge, so as to achieve mutual separation of the printed motifs. As the inking roller rolls on the printing form of the printing form carrier and the counterpressure roller with the printed material rolls on the printing form carrier, vibrations are created when the run-up edge and/or the run-off edge come/comes into contact with the inking roller and the counterpressure roller. The pulses are inevitably created, since the inking roller has to roll on the printing form under a certain contact pressure so as to ensure a uniform inking process.
It is usual in the case of printing units of the prior art for the rotation axes of the roller pairs which are in each case in mutual contact to be disposed so as to be mutually parallel. The two axes of a roller pair here are in a common plane, wherein it is known for the rotation axes of a roller pair in the common plane to be set so as not to be mutually parallel, so as to adjust the respective nip between the rollers at one end to be different from the nip at the other end, the nip between the cylinders thus opening out or tapering off.
The transfer of ink in the first nip is performed while the inking roller is in contact with the raised areas of the printing form carrier. The transfer of ink in the second nip is performed while the inked and raised areas of the printing form carrier are in contact with the substrate which is supported by the substrate-bearing roller or counterpressure roller. On account of the contact forces between the individual rollers, which arise as a line load, said rollers are elastically deformed, on account of which the nip between the rollers at the clamping points is longer than in the center of the rollers.
As the raised regions of the printing form pass through the first nip and the second nip, the former are squeezed across the entire working width of the printing unit at least in part regions, so as to ensure that ink is transferred in a sufficient manner. The quantity of the ink being transferred inter alia depends on the prevailing squeezing forces. As a consequence of squeezing and rotating, rising and ebbing contact forces are created between the associated rollers. At a high rotating speed of the rollers and in the case of an unfavorable location of the raised regions of the printing form, the rising and ebbing of the contact forces is perceivable as an action of force or an internal impact load arising in a pulsed manner. There is a particularly unfavorable effect here that in practice printing forms which have an edge of the raised elements which runs parallel to the longitudinal extent of the rotation axes and which is printed as an image edge which runs transversely to the running direction of the substrate are often employed. These are the so-called run-up or run-off edges. There is furthermore a negative effect that in practice printing forms having raised regions often have a thickness profile across the working width of the press, so that in order for operational contact to be guaranteed the printing form or the printing plate has to be squeezed in such a manner that also those raised regions that have the lowest elevation still have operational contact. Consequently, the comparatively thick regions of the printing form are excessively squeezed, further increasing the contact forces arising within the nips.
Vibrations which are extraordinarily disturbing are created when a single run-up edge is present across the entire printing width of the printing form, that is to say in the case of a maximum press format, in other words when one or a plurality of printed images having a common run-up and run-off edge are located on the printing form carrier. Such vibrations are clearly displayed as lines in the printed image at considerably high machine speeds and/or at high printing contact pressure and, in particular, in the case of a long run-up edge or run-off edge which is parallel with the cylinder axis of the printing form carrier, and are counteracted by adapting the machine speed. Alternatives to reducing the machine speed include substituting another printing form material or using a shock-absorbing substructural material which is applied between the printing form carrier and the printing form. This substructural material in the form of an adhesive film with damping properties serves in fixing the printing form, that is the so-called printing plate, to the printing form carrier.